Telomeres cap and protect the ends of all human chromosomes. In healthy adult tissue, telomeres shorten with each round of cell division as part of the normal aging process. This mechanism limits human cells to a finite number of cell divisions before induction of programmed cell death and therefore serves as a tumor suppressor. In cancer cells, however, an enzyme called telomerase is upregulated to nullify the limited number of cell divisions. As such, cancer cells are capable of infinite division, which allows proliferation of ~90% of all malignant tumors. Due to this unique and critical role in cancer biology, telomerase provides a novel target for innovative therapeutics. The purpose of the present proposal is to explore a novel mechanism toward using telomerase as an anti-cancer target. In our approach, we will design non-native nucleotide analogs to be preferentially and selectively incorporated by telomerase into telomere DNA. Once incorporated, the non-native nucleotides should abrogate binding of essential telomere-end binding proteins such as the Protection of Telomeres 1 (POT1), which is highly specific for telomere DNA sequence. Abrogation of POT1 binding induces cell death through a cascade of DNA damage events. The cell-killing potential of these non-native nucleotide compounds will be validated by measuring their potency and selectivity against telomerase-positive cancer cell lines as well as xenograft mouse models. We predict that our strategy will provide a selective mechanism to potentially treat a wide range of human cancers.